A variety of surface light source devices that are used to illuminate transmission type liquid crystal displays or the like from their rear have been proposed and put into practical use. Such surface light source devices are broadly classified into the edge light type and the direct type according to the mode of conversion of a non-surface light source to a surface light source.
Of these surface light source devices, a surface light source device of direct type is so constructed that light is introduced to the rear of a transmission type display device serving as a light valve, such as an LCD panel, directly from cathode ray tubes (arc tubes) arranged in parallel. In such a surface light source device of direct type, the cathode ray tubes and the transmission type display device are properly spaced from each other. In this space, there are placed a diffusing sheet and a combination of two or more optical sheets capable of converging light.
Such a conventional surface light source device of direct type is poor in the light-converging properties, although the number of required optical sheets is large. In order to solve this problem, the structure of the transmission type display device itself, such as an LCD panel, has been improved so that even when light is obliquely incident on the display device, an image of excellent quality can be produced. However, this conventional way of improvement suffers reduction in optical efficiency, and also suffers increase in cost because of the complicated structure of transmission type display device such as an LCD panel.
Another disadvantage of the conventional surface light source device of direct type has been that the light intensity (luminance) on the display device tends to be non-uniform depending on the distance from the cathode ray tubes (i.e., whether a certain point on the display device is close to the cathode ray tube or to the space between the cathode ray tubes that are arranged in parallel).
A possible method of avoiding the above-described non-uniformity is to place the cathode ray tubes and the transmission type display device such as an LCD panel sufficiently apart from each other. This method has been disadvantageous in that the display has an increased total thickness.
Another possible method of avoiding the above-described non-uniformity is to increase the degree to which the optical sheets or the like that are placed between the cathode ray tubes and the transmission type display device such as an LCD panel diffuse light, or to control the amount of light which the optical sheets transmit. This method has been disadvantageous in that it causes decrease in the amount of usable light.
More specifically, Japanese Patent Laid-Open Publications No. 119703/1993 and No. 242219/1999, for example, propose a method for maintaining the uniformity of light, in which a light-shielding member such as a lighting curtain or a light-shielding dot layer is provided in a surface light source device. This method has been disadvantageous in that it causes decrease in the amount of usable light, similar to the above-described method.
Further, Japanese Patent Laid-Open Publication No. 347613/1994 proposes a method in which, in a surface light source device, the diffusion of light in two directions is controlled by a sheet having lenticular lenses on both surfaces. Since the function of converging light cannot be sufficiently obtained by this method, the optical axis on every area on the face of the transmission type display device such as an LCD panel changes according to the position of the area relative to the cathode ray tubes. Thus, this method still has been disadvantageous in that the brightness of light on the display screen varies depending on the position from which the display screen is observed.
Further, Japanese Patent Publication No. 2002-535690 proposes a method of using a prism that mainly refracts and emits light that has perpendicularly entered a sheet, in combination with a prism that mainly, totally reflects light that has perpendicularly entered the sheet and then emits the light. In the method disclosed in Japanese Patent Publication No. 2002-535690, almost all the light that has perpendicularly entered the sheet is made to emerge therefrom to the light-emerging surface side, in both the case in which the light is refracted and emitted, and the case in which the light is totally reflected and then emitted. Thus, the totally reflected light, in particular, emerges from the sheet at a large angle to widely spread. Therefore, it is difficult to focus such light that emerges from the sheet at angles within a required range.